Conventional thermal flow measuring devices use usually two (embodied to be as similar as possible) temperature sensors, which are arranged in (most often pin-shaped) metal-shell housings—so-called stingers—and which are in thermal contact with the medium, which is flowing through a measuring tube or through the pipeline. For industrial application, the two temperature sensors are usually installed in a measuring tube; the temperature sensors can, however, also be directly mounted in the pipeline. One of the two temperature sensors is a so-called active temperature sensor, which is heated by means of a heating unit. For this heating unit, either an additional resistance heating is provided, or the temperature sensor itself is a resistance element, e.g. an RTD (Resistance Temperature Device) sensor, which is heated through conversion of an electrical power, for example, through a corresponding variation of the electrical measuring current. The second temperature sensor is a so-called passive temperature sensor: It measures the temperature of the medium.
In a thermal flow measuring device, the heatable temperature sensor is usually heated in such a way, that a fixed temperature difference is set between the two temperature sensors. Alternatively, it has also been known to supply a constant heating power via a control unit, which may utilize either open or closed loop control.
If there is no flow in the measuring tube, an amount of heat, which is constant in time, is then required for maintaining the predetermined temperature difference. If, in contrast, the medium to be measured is moving, the cooling of the heated temperature sensor is essentially dependent on the mass flow of the medium flowing past. Since the medium is colder than the heated temperature sensor, heat is transported away from the heated temperature sensor by the flowing medium. Thus, in the case of a flowing medium, in order to maintain the fixed temperature difference between the two temperature sensors, an increased heating power is required for the heated temperature sensor. The increased heating power is a measure for the mass flow of the medium through the pipeline.
If, in contrast, a constant heating power is fed in, the temperature difference between the two temperature sensors then decreases as a result of the flow of the medium. The particular temperature difference then serves as a measure for the mass flow of the medium through the pipeline (or through the measuring tube).
There is, thus, a functional relationship between the heating energy needed for heating the temperature sensor, and the mass flow through a pipeline or through a measuring tube. In thermal flow measuring devices, the dependence of the heat transfer coefficient on the mass flow of the medium through the measuring tube (or through the pipeline) is utilized for determining the mass flow. Devices which operate according to this principle are available from and sold by the assignee under the name “t-switch”, “t-trend” or “t-mass”.
Until now, primarily RTD-elements with helically wound platinum wires have been employed in thermal flow measuring devices. In the case of thin-film resistance thermometers (TFRTDs), a meander-shaped platinum layer is conventionally vapor deposited on a substrate. Over this, a glass layer is applied for protection of the platinum layer. The cross section of the thin-film resistance thermometers is rectangular, in contrast to the RTD-elements, which have a round cross section. Heat transfer into the resistance element and/or out of the resistance element accordingly occurs via two surfaces lying opposite each other, which, together, make up a large part of the total surface of a thin-film resistance thermometer.
In U.S. Pat. Nos. 6,971,274 and 7,197,953, installation of a cuboid-shaped thin-film resistance thermometer in a round pin-shaped housing is achieved in the following way. In a spacer socket (made of metal) with a rectangular recess, the thin-film resistance thermometer is applied in such a way, that at least the two large surfaces of the thin-film resistance thermometer (which lie opposite each other) have virtually gap-free contact with the surfaces of the spacer socket lying opposite them. To this effect, the spacer socket has a rectangular recess, which is manufactured according to the outer dimensions of the thin-film resistance thermometer. The spacer socket should hold the thin-film resistance thermometer tightly. In this regard, the spacer socket and the thin-film resistance thermometer virtually form a press fit. The spacer socket itself and the pin-shaped housing likewise form a press fit. In this way, use of a potting compound or some other fill material is made unnecessary. The advantage of this construction is that, due to the spacer socket, a good heat transfer exists between the thin-film resistance thermometer and the measured medium on all sides. However, due to the fixed seating of the thin-film resistance thermometer and/or through different coefficients of thermal expansion for the participant materials, mechanical stresses arise in the thin-film resistance thermometer.